The air actuating systems for tractor-trailer combination vehicles of the day comprise a number of individual valves that perform various functions in controlling the flow of pressure from an air compressor on the tractor to brake actuating pressure chambers at the various wheels of the combination vehicle. It will be understood that a great number of types of hazards can occur in combination vehicles of this type, and that various portions of the valving in such systems will protect against one or more of such hazards. One hazard which exists is caused upon the separation of the tractor and trailer of the combination vehicle; which, of course, will vent both the control and supply pressure lines coming from the tractor so that the brakes on both portions of the vehicle would be disabled if suitable valving were not supplied. Other functions which are performed by the valving of such systems will protect against a loss in pressure of one of the control or supply lines.
Many brake actuating cylinders include springs which mechanically apply the brakes and an opposing diaphragm against which service pressure is supplied to remove the spring applying force. These brakes will also have a second diaphragm against which control pressure is supplied for normal actuation of the brakes. It is possible that failures of some components will cause both the springs and the control diaphragm to apply their forces simultaneously to produce a force that might overload the mechanical brake components. There is considerable need in such systems for valve construction having improved reliability and service life.
Another object of the invention is the provision of new and improved valving having two inlet ports and arranged to pass on pressure from whichever inlet port is supplying the greatest pressure, while closing off the other of the inlet ports.
Apart from the general need for improved valve construction for passing pressure from either of two inlet ports to an outlet port, it is an object to provide an improved valve construction for passing an initial supply pressure to the spring brake releasing chamber, for thereby allowing the supply pressure to fall to a predetermined value before causing the spring brakes to be applied, and for causing the spring brakes to be applied only when the supply pressure falls below such predetermined value.
A still further object of the present invention is the provision of new and improved valving which will perform both of the above-mentioned functions.
Further objects and advantages of the invention will become apparent to those skilled in the art to which the invention relates from the following description of the preferred embodiments described with reference to the accompanying drawings.